Viscosity Index Improvers (VIIs) are additives that help maintain the viscosity of lubricating oils across a wide temperature range, ensuring consistent performance.

This article will explore the nature of viscosity index improvers and their role in industrial and automotive lubricants. We will also look at their impact on lubricant efficiency, innovations involving this type of additive, and future trends.

Before discussing the nature of viscosity index improvers, we need to understand the role of viscosity. Essentially, this is one of the most critical functions of a lubricant, as it directly affects its flow rate and ability to keep the two interacting surfaces apart.

By nature, all base oils have an assigned viscosity based on their blend. However, other properties are required when we’re creating finished industrial or automotive lubricants. For instance, we may need the oil to withstand higher temperatures while still maintaining a particular viscosity, which not only provides wear protection for the equipment but also flows at a rate that does not incur frictional losses. Those are a lot of functions!

Typically, as temperature increases, viscosity decreases, and as the temperature decreases, the viscosity increases. One example is the state of water: when heated, it can turn into a gas (lower viscosity), or when frozen, it can transform into ice (higher viscosity). However, depending on the type of material, there will be varying rates of viscosity change with temperature. The viscosity/temperature relationship is called the viscosity index (VI).

As per Mortier, Fox, & Orszulik (2010), the kinematic viscosity of oil is measured at 40°C and then at 100°C. The viscosity change is then compared with an empirical reference scale initially based on two sets of crude oils: a Pennsylvania crude arbitrarily assigned a VI of 100 and a Texas Gulf crude assigned a VI of 0.

The higher the VI, the less effect that temperature has on the oil, which means that the oil can maintain a particular viscosity for a longer time at a more extensive temperature range. This is ideal for lubricants in environments experiencing temperature changes. However, not all oils have a high viscosity index. Typically, paraffinic oils can have a very high viscosity index. On the other hand, naphthenic oils have a low or medium viscosity index. The table below gives an overview of the viscosity index for various oils.